JavaScript - constants

In this article, we would like to show you how to declare and use constants using the const keyword in JavaScript.

Syntax

const myConstant = value;

Description

Constants are declared using the const keyword and they must be initialized. They are block-scoped such as variables declared using the let keyword. The constant values can't be changed through reassignment, and they can't be redeclared. However, if you create a constant object or array its properties or items can be updated or removed.

Practical examples

Example 1

In this example, we create a constant and try to reassign a value to it.

// ONLINE-RUNNER:browser;

const myConstant = 1;

try {
  myConstant = 2;
} catch (error) {
  console.log(error); // TypeError: Assignment to constant variable.
}

console.log('myConstant = ' + myConstant); // 1

Output:

TypeError: Assignment to constant variable.
number = 1

Example 2

In this example, we create an uninitialized constant.

// ONLINE-RUNNER:browser;

const myConstant;

Output:

Uncaught SyntaxError: Missing initializer in const declaration

Example 3

This example shows, that constants are block-scoped.

// ONLINE-RUNNER:browser;

const myConstant = 3;

if (true) {
  const myConstant = 2;
  console.log(myConstant); // Output: 2
}

console.log(myConstant); // Output: 3

Output:

2
3

References

1. const - JavaScript | MDN